Existing cellular network systems have enjoyed great popularity in recent years. At various times, the channels that carry the voice communications of the cellular system may be idle (i.e. no signal transmission over the channel at a particular time). These unused or idle voice channels may be utilized for other communication such as data communication. In particular, an overlay system network utilizing unused or idle voice channels for digital data communication is desirable. Cellular Digital Packet Data (CDPD) is such an overlay system which provides mobile datagram service utilizing the structure of existing cellular telephone networks. The CDPD system allows digital data transmission over idle channels of an already existing cellular system. A consortium of cellular communication carriers prepared and released in 1993 a specification entitled "Cellular Digital Packet Data System Specification."
The specification defines a protocol to be used by the industry for transmitting and receiving data messages over an existing cellular communication system. The protocol specifies the format of the data message. More particularly, the data message has a preamble formed by a dotting sequence of 38 bits in length followed by a synchronization pattern of 22 bits in length. Following the preamble is the data sequence comprised of n multiples of 385 bits of data. The dotting sequence is an alternating series of 1's and 0's. The synchronization pattern has the following bit pattern 1011 1011 0101 1001 1100 00.
The CDPD overlay system utilizes the facilities of the existing cellular system to transmit data. In particular, a plurality of remote subscriber units communicate with other remote subscriber units through base stations. The data communication from the remote subscriber units to the base stations is wireless.
Several variable factors are introduced into the signals transmitted from a remote subscriber unit to the base unit which, if not correctly compensated for, may lead to incorrect demodulation of the data signal by the base station. One variable factor is the frequency offset of the transmitted signal. Frequency offset is introduced by several factors. Each remote subscriber unit utilizes a crystal oscillator to provide the proper carrier frequency on which the data signal is transmitted, however, the accuracy of the crystal oscillator from one remote subscriber unit to the next may vary thereby introducing an unknown frequency offset in the transmitted signal. In addition, because the remote subscriber units are often used in conjunction with moving objects such as automobiles, a doppler frequency shift is introduced in the transmitted signal which is dependent upon the speed of the moving object. In addition, the time of arrival of the data sequence is dependent upon the distance the remote subscriber unit is from the base station.
The transmitted signal also suffers from an additional impairment which is called frequency drift. The frequency drift during the beginning of the transmitted signal is known as "load pull" or "key-up transient." This key-up transient severely degrades the signal quality often introducing more than 3 KHz frequency drifts during the dotting sequence which makes it almost impossible to estimate reliably the frequency offset using the dotting sequence. FIG. 1 illustrate the signal amplitude profile and FIG. 2 illustrates the carrier frequency profile for a transmitted signal under ideal conditions, i.e., within a specified frequency offset tolerance. FIG. 3 illustrates the carrier frequency profile for a typical transmittal signal from a remote subscriber unit in the CDPD system. As can be seen from the carrier frequency profile in FIG. 3, the frequency offset during the dotting sequence varies greatly and is typically outside of the specified tolerance. Thus, in such a case, the synchronization pattern may be used to estimate the frequency offset. This, however, involves an unacceptable acquisition delay for a CDPD demodulator implemented on a low cost digital signal processor because of the computational burden involved. The frequency offset f.sub.offset of FIGS. 2 and 3 refers to a steady state frequency offset.
Thus, in order for the base station to properly demodulate a signal transmitted by a remote subscriber unit, the base station must estimate the frequency offset introduced in the transmitted signal as well as estimate the time of arrival of the data sequence. The frequency offset and time of arrival may be referred to as data acquisition parameters.
One method of estimating frequency offset and time of arrival is to perform a correlation of the received signal with the synchronization pattern, frequency and time shifted according to frequency offset and timing resolution tolerances. More particularly, equation (1) below describes the correlation that may be used to estimate frequency offset and time of arrival. EQU c(.tau..sub.k, f.sub.m)=.intg.r(t-.tau..sub.k).multidot.S*(t)e.sup.-j2.pi.f.sbsp.m.sup.t dt, where (1) EQU k=0, . . . 47 EQU m=0, . . . 20
and r(t) is the received signal, S*(t) is the complex conjugate of the synchronization pattern, .tau..sub.k is the estimated time of arrival and f.sub.m is the estimated frequency offset. The variables k and m have been chosen for the CDPD system which allows.+-.3 KHz frequency offset and up to 12 bits of timing uncertainty. Within these tolerances, if one wanted to resolve the frequency offset to a 150 Hz accuracy (21 frequency bins, i.e., m=0, . . . 20) and timing up to one eighth of bit time (48 time bins, i.e., k=0, . . . 47), 960 correlation computations must be performed. More particularly, a two-dimensional array as shown in FIG. 4 would need to be searched.
Equation (1) can be rewritten in a sampled version format as equation (2) below: ##EQU1## where T.sub.s is the sampling rate. As is well known to those of ordinary skill in the art, .tau..sub.k and f.sub.m are scanning parameters which are adjusted to reveal the relatedness or correlation between the functions.
Equation (1) or (2) is solved to find the values .tau..sub.k and f.sub.m which maximize equation (3) below. ##EQU2##
Thus as shown in FIG. 4 a two dimensional time-frequency search is performed where a matrix of 21 frequency bins and 48 time bins need to be searched to estimate the frequency offset f.sub.m and time of arrival .tau..sub.k. Such a search is computationally burdensome which presents a problem for real-time systems. In addition, because of the number of computations involved, more expensive digital signal processing circuitry must be used.
An object of the present invention is to reduce the computational burden in acquiring data acquisition parameters of a CDPD signal which has significant impairment due to the "key up transient" previously described. It is another object of the present invention to allow the data acquisition parameters, in particular frequency offset and time of arrival, to be acquired quickly so that the system will operate efficiently in a real-time environment. In addition, the present invention to reduce the number of computations performed so that less expensive digital signal processors may be used thereby reducing the cost of the overall system.